1. Field of the Invention
The present invention relates to copying machines, printers, facsimiles, and color image forming apparatuses employing the electrophotographic method having these functions, and in particular to color image forming apparatuses that have an intermediate transfer body and in which color toner images are superimposed on that intermediate transfer body.
2. Description of Related Art
Color image forming apparatuses of the electro-photographic method have been known which employ an intermediate transfer body, the toner image formed on the image supporting body which is the photosensitive body is transferred to the intermediate transfer body, and the toner image on that intermediate transfer body is transferred onto the transfer medium (also called recording paper or simply paper). In such a color image forming apparatus, after the toner images formed successively on the image supporting body and charged to a specific electrostatic voltage are first transferred one toner image upon the other onto the intermediate transfer body using electrostatic force, the toner images on that intermediate transfer body is collectively transferred onto the transfer medium using electrostatic force.
Since the amount of charge per toner particle is almost uniform, the potential on the intermediate transfer body is determined by the amount of toner adhered within a specific area, the electric surface potential of the part of the intermediate transfer body of a color image forming apparatus in which toners of several colors have been superimposed will be larger than the electrostatic surface potential of the area in which toner of only one color is adhered.
When the variation in the potential is large on the intermediate transfer body after primary transfer, it becomes easy for different types of image defects to occur at the time of second transfer and methods in which the electrostatic charge of the toner on the intermediate transfer body is made uniform thereby carrying out uniform second transfer have been proposed in, for instance, Japanese Unexamined Laid-Open Patent No. Hei 10-274892 and Japanese Unexamined Laid-Open Patent Publication No. Hei 11-143255.
In these Japanese Patent Publications, since the quantity of electrostatic charge of the toner on the intermediate transfer body has been made uniformly large in a low humidity environment or when the resistance of the paper is high, image defects due to electrostatic discharge caused by increase in the potential of the paper are more likely to occur at the time of the second face image transfer in the both-sides copy mode, and if the transfer voltage is made low in order to avoid such image defects there will be insufficient transfer electric field in the part of the toner layer with large total electrostatic charge thereby causing density variations or toner scattering around the edges of the image.
In order to prevent transfer defects due to too large total electrostatic charge of the toner layer, although, prior technology exists in which a uniform discharge is made to the polarity opposite to that of the toner image potential for all the image areas using corona discharge, but in this case the toner image potential in the part with large amount of toner adhesion has been suppressed in an appropriate value thereby obtaining satisfactory images, the toner image potential would be too low in the image area with low quantity of toner adhesion such as half-tone areas, and in some cases the potential could get reversed thereby causing image defects such as image roughness, toner scattering, etc.